Electromagnetic surveying for hydrocarbon reservoirs

ABSTRACT

An electromagnetic survey method for surveying an area that potentially contains a subterranean hydrocarbon reservoir. The method comprises detecting a detector signal in response to a source electromagnetic signal, resolving the detector signal along at least two orthogonal directions, and comparing phase measurements of the detector signal resolved along these directions to look for a phase separation anomaly indicative of the presence of a buried hydrocarbon layer. The invention also relates to planning a survey using this method, and to analysis of survey data taken using this survey method. The first and second data sets may be obtained concurrently with a single horizontal electric dipole source antenna. The method is also largely independent of a source-detector pair&#39;s relative orientation and so provides for good spatial coverage and easy-to-perform surveying.

BACKGROUND OF THE INVENTION

The invention relates to seafloor electromagnetic (EM) surveying for oiland other hydrocarbon reserves.

Geophysical methods for mapping subterranean resistivity variations byvarious forms of EM surveying have been in use for many years [1, 2, 3,10]. In these methods, electric field detectors are placed on theseafloor at carefully chosen positions at ranges up to about 10 km froman electromagnetic source. Detector signals measured at the detectorsare sensitive to variations in subterranean strata configurationresistivity beneath the area being surveyed. However, EM surveying wasnot widely thought of as a technique that could be applied to findinghydrocarbon reservoirs.

More recently, it was proposed to use EM surveying to find hydrocarbonreservoirs. An early proposal by Statoil was to use the vertical currentflow components to detect hydrocarbon layers [4, 5], since it is thesecomponents that are sensitive to the presence of a thin resistive layer.This was based on the understanding that a subterranean strataconfiguration that includes a resistive hydrocarbon layer embeddedwithin less resistive sediments will give rise to a measurableenhancement of the electric field amplitude compared to a subterraneanstrata configuration comprising only water-bearing sediments. TheStatoil proposal was to collect data from detector locations which arein-line with (i.e. end-on to) the axis of a horizontal electric dipole(HED) antenna so that the galvanic mode, that should be most sensitiveto the presence of a buried high resistivity layer, dominates. However,it was established that the Statoil method could not provide reliableresults, since the in-line data collected is incapable of distinguishingbetween a thin buried hydrocarbon layer of high resistivity situated inless resistive strata, on the one hand, and a non-hydrocarbon bearingrock formation in which the strata exhibits increasing resistivity withdepth, on the other hand, the latter being a common feature of manylarge scale sedimentary structures.

It was then proposed to use the EM surveying method according to Sinha[12] for finding hydrocarbon reservoirs [9, 13] and it was thenconfirmed that this method works well in practice for findinghydrocarbon reservoirs [6, 7]. The essence of the Sinha method is tonormalise the in-line data with equivalent data for the samesource-detector pair locations collected in an orthogonal geometry wherethe inductive mode dominates the response, referred to as a broadsidegeometry. In the broadside geometry the axis of the HED dipole antennaof the source is perpendicular to a line between the detector andsource. EM surveying of a hydrocarbon reservoir applying the Sinhamethod is now described in more detail.

Survey data is collected by using a surface vessel to tow a submersiblevehicle carrying a HED antenna over a survey area. The HED antennabroadcasts a source electromagnetic signal into the seawater. Detectorsare located on the seafloor over the survey area and measure a signal inresponse to EM fields induced by the HED antenna. The amplitude of thedetector signals is sensitive to resistivity variations in theunderlying strata configuration and this is used to determine the natureof the subsea structure. In order to successfully map subterraneanresistivity variations, the orientation of the current flows induced bythe source electromagnetic signal must be considered [6]. The responseof seawater and subterranean strata to the source electromagnetic signalis different for horizontally and vertically flowing induced currentcomponents. For horizontally flowing current components, the couplingbetween the layers comprising the subterranean strata is largelyinductive. This means the presence of a thin resistive layers (which isindicative of a hydrocarbon layer) does not significantly affect thedetector signal at the seafloor since the large scale current flowpattern is not affected by the thin resistive layer. On the other hand,for vertical current flow components, the coupling between layers islargely galvanic (i.e. due to the direct transfer of charge). In thesecases even a thin resistive layer strongly affects the detector signalsat the seafloor since the large scale current flow pattern isinterrupted by the resistive layer.

While it is the vertical components of induced current flow which aremost sensitive to the presence of a thin resistive layer, sole relianceon these components for detecting a hydrocarbon layer is not possiblewithout ambiguity. The effects on the amplitude signals at the detectorsarising from the presence a thin resistive layer can beindistinguishable from the effects which arise from other realisticlarge scale subterranean strata configurations. In order to resolvethese ambiguities, it is necessary to determine the response of thesubterranean strata to both horizontal (i.e. inductively coupled) andvertical (i.e. vertically coupled) induced current flows [6].

An electromagnetic source such as a HED antenna generates both inductiveand galvanic current flow modes, with the relative strength of each modedepending on source-detector geometry. At detector locations which arebroadside to the HED antenna dipole axis, the inductive mode dominatesthe response. At detector locations which are in-line with the HEDantenna dipole axis, the galvanic mode is stronger [6, 8, 9, 10].Accordingly, the response of the subterranean strata to vertical inducedcurrent flows along a line between a source location and a detectorlocation is determined by arranging the HED antenna to present an end-onorientation to a detector, and the response of the subterranean stratato horizontal induced current flows along the line between the sourcelocation and the detector location is determined by arranging the HEDantenna to present a broadside orientation to the detector. Data fromboth geometric configurations is required.

It is therefore important when designing a practical EM survey fordetecting buried hydrocarbon layers using known techniques todistinguish between source and detector configurations in which thecoupling between layers is largely inductive due to horizontal currents(in which case the survey has little sensitivity to the presence of athin resistive layer) and those in which the coupling between layers islargely galvanic due to vertical currents (in which case blocking of thepassage of this current flow by a reservoir leads to a survey which isstrongly sensitive to the presence of a thin resistive layer).

FIG. 1 shows in plan view an example survey geometry according to theSinha method. There are sixteen detectors 25, and these are laid out ina square grid on a section of seafloor 6 above a subterraneanhydrocarbon reservoir 56. The hydrocarbon reservoir 56 has a boundaryindicated by a heavy line 58. The orientation of the hydrocarbonreservoir is indicated by the cardinal compass points (marked N, E, Sand W for North, East, South and West respectively) indicated in theupper right of the figure. To perform a survey, a source such as a HEDantenna, starts from location ‘A’ and is towed along a path indicated bythe broken line 60 through location ‘B’ until it reaches location ‘C’,which marks the end of the survey path. As is evident, the tow pathfirst covers four parallel paths aligned with the North-South directionto drive over the four “columns” of the detectors. This portion of thesurvey path moves from location ‘A’ to location ‘B’. Starting fromlocation ‘B’, the survey path then covers four paths aligned with theEast-West direction which drive over the four “rows” of detectors. Eachdetector is thus passed over in two orthogonal directions. The survey iscompleted when the source reaches the location marked ‘C’.

During the towing process, each of the detectors 25 presents severaldifferent orientation geometries with respect to the source. Forexample, when the source is directly above the detector position D1 andon the North-South aligned section of the tow path, the detectors atpositions D5, D6 and D7 are at different ranges in an end-on position,the detectors at positions D2, D3 and D4 are at different ranges in abroadside position and the detector at positions D8 and D9 are midwaybetween. However, when the source later passes over the detectorposition D1 when on the East-West aligned section of the tow path, thedetectors at positions D5, D6 and D7 are now in a broadside position,and the detectors at position D2, D3 and D4 are in an end-on position.Thus, in the course of a survey, and in conjunction with the positionalinformation of the source, data from the detectors can be used toprovide details of the source electromagnetic signal transmissionthrough the subterranean strata for a range of distances andorientations between source and detector. Each orientation providesvarying galvanic and inductive contributions to the signal propagation.In this way the continuous towing of the source can provide a surveywhich samples over the extent of the subterranean reservoir.

The Sinha method has been demonstrated to provide good results inpractice. However, it has some limitations.

Firstly, since the two modes cannot be easily separated there willgenerally be a level of cross-talk between them at a detector and thiscan lead to ambiguities in the results.

Secondly, in order to obtain survey data from both in-line and broadsidegeometries, the HED antenna needs to present two different orientationsat each source location. This requires the surface vessel to makemultiple passes over broadcast locations and can lead to long andcomplex tow path patterns.

Thirdly, the survey can only provide the best data possible at discretesource locations. This is because of the geometric requirements of a HEDantenna survey which dictate that, at any point during the survey, datacan only be optimally collected from those detectors to which the HEDantenna is arranged either in-line or broadside. At other orientations,separation of the inductively and galvanically coupled signals becomesmore difficult, and resulting data are less reliable. For instance,referring to the figure, when the HED antenna is at a point on the towpath directly above the detector marked D1 and on the North-Southaligned section of the tow path, in-line data can only be collected fromthe detectors marked D5, D6 and D7, whilst broadside data can only becollected from the detectors marked D2, D3 and D4. The other detectors(for example those marked D8, D9 and D10) provide only marginally usefulinformation at this point of the survey because of the complex mixing ofthe galvanically and inductively coupled modes. Furthermore, if, forexample, the HED antenna is at the location identified by referencenumeral 57 in the figure, which is on a North-South aligned section ofthe tow path, in-line data can be collected from the detectors markedD3, D8, D9 and D10, but broadside data cannot be collected from any ofthe detectors. Since both broadside and in-line data are required foroptimal analysis, the best data possible with the square detector arrayshown in the figure can only be collected from points along the tow pathwhere the source is directly above one of the detector locations.

In summary, with the Sinha method, the time during which good qualitydata can be collected represents only a small fraction of the overalltime taken to perform a survey. Furthermore, in addition to the surveybeing time-inefficient, it is necessary to accurately follow a complextow path which has to complement the detector layout, and the detectorsthemselves must also be carefully accurately arranged. The difficultiesin controlling both the position and the orientation of a towed sourceantenna, coupled with this need to accurately follow a particular towpath relative to the detector grid, is one of the major sources of errorin surveys of these kind. The disadvantages associated with the surveyconstraints imposed by the Sinha method are the price to pay forresolving the ambiguities inherent in the Statoil method.

SUMMARY OF THE INVENTION

According to the invention there is provided an electromagnetic surveymethod for surveying an area that is thought or is known to contain asubterranean hydrocarbon reservoir, comprising: transmitting a sourceelectromagnetic signal from a source location; detecting a detectorsignal at a detector location in response thereto; and obtaining surveydata indicative of phase difference between first and second componentsof the detector signal resolved along first and second directionsrespectively.

By comparing phase measurements of different components of the detectorsignal, a phase separation anomaly can be detected which is sensitive tothe presence of a hydrocarbon layer or reservoir within a subterraneanstrata configuration. The presence or not a phase separation anomaly,and hence the presence or not of a hydrocarbon layer, can be determinedwith a single source orientation. There is no need, as there is withknown methods based on amplitude, for data to be collected withdifferent source orientations. Accordingly, surveys can be performedmore quickly and without needing to accurately control the sourceorientation. Furthermore, because of this insensitivity of a phasemeasurement to the relative source orientation, reliable data collectionis not limited to specific source location and detector locationgeometries, as is the case when collecting in-line/broadside amplitudedata, and a much less complex and geometrically restrained towpath canbe employed to survey an extended area.

The first and second components can be any two of radial, vertical andazimuthal. The clearest phase anomaly appears to occur from the pairingof radial and azimuthal components. It is also possible to use all threecomponents together, i.e. to have first, second and third components.

The first, second and, if used, third directions are preferablyorthogonal, since by observing geometrically independent components ofthe detector signals, there is minimal cross-talk between the first andsecond data sets, and the sensitivity to the presence of a hydrocarbonreservoir is accordingly increased.

The source electromagnetic signal can be broadcast from an antennamounted on a submersible vehicle, or from a static location, such aswithin a borehole, or from an oil or gas platform.

The source electromagnetic signal can be emitted at differentfrequencies to obtain survey data at a plurality of differentfrequencies. Moreover, the source electromagnetic signal can be emittedat a variety of frequencies, preferably between 0.01 Hz and 10 Hz. Themethod can be advantageously repeated over the same survey area usingdifferent frequencies of source electromagnetic signal. Lowerfrequencies are generally preferred. By probing the subterranean strataat a number of different frequencies of source electromagnetic signal,it is possible to obtain improved vertical resolution of structureswithin the subterranean strata configuration.

The source signal can be from a horizontal electric dipole. Such asignal can be provided using existing equipment, and also allowsrelatively simple inversion modelling.

The invention also provides a method of analysing results from anelectromagnetic survey of an area that is thought or known to contain asubterranean hydrocarbon reservoir, comprising: providing survey dataindicative of phase difference between first and second components of adetector signal resolved along first and second directions respectively;extracting the phase differences from the survey data; and determining ametric from the phase differences that is predictive of the presence orabsence of hydrocarbon.

The phase differences can be extracted by rotationally transforming thesurvey data from an instrument frame to a source frame.

The invention also provides a computer program product bearing machinereadable instructions for implementing the analysis method.

The invention further provides a method of planning an electromagneticsurvey of an area that is thought or known to contain a subterraneanhydrocarbon reservoir, comprising: creating a model of the area to besurveyed including a seafloor, a rock formation containing a postulatedhydrocarbon reservoir beneath the seafloor, and a body of water abovethe seafloor; setting values for depth below the seafloor of thepostulated hydrocarbon reservoir and resistivity structure of the rockformation; and performing a simulation of an electromagnetic survey inthe model to obtain from the model phase differences between first andsecond components of a detector signal resolved along first and seconddirections respectively.

Repeated simulations for a number of distances between a source and adetector and frequencies can be performed in order to allow optimumsurveying conditions in terms of source-to-detector distance andfrequency of EM signal for probing the hydrocarbon reservoir to beselected when performing an electromagnetic survey. The effects ofdiffering detectors array configurations and source tow paths can alsobe modelled.

The invention also provides a computer program product bearing machinereadable instructions for implementing the planning method.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same maybe carried into effect reference is now made by way of example to theaccompanying drawings in which:

FIG. 1 is a schematic plan view showing an example survey geometryfollowing prior art principles in which sixteen detectors are laid outon a section of seafloor above a subterranean reservoir;

FIG. 2A shows in schematic vertical section a surface vessel undertakingan EM survey;

FIG. 2B is a plan view detailing a polar coordinate system;

FIG. 3 shows in schematic vertical section a model uniform backgroundsubterranean strata configuration;

FIG. 4 shows in schematic vertical section a model hydrocarbon-layersubterranean strata configuration;

FIG. 5A shows a graph plotting calculations of phases of differentcomponents of detector signals seen during a model electromagneticsurvey of the subterranean strata configurations shown in FIGS. 3 and 4;

FIG. 5B shows a graph plotting differences in the phases shown in FIG.5A;

FIG. 6 shows in schematic vertical section a model of a non-hydrocarbonbearing subterranean strata configuration;

FIG. 7 shows a graph plotting calculations of phases of differentcomponents of detector signals seen during a model electromagneticsurvey of the subterranean strata configurations shown in FIGS. 3, 4 and7;

FIG. 8 shows a graph plotting differences in the phases shown in FIG. 7;

FIG. 9A is a schematic plan view showing an example survey geometryaccording to an embodiment of the present invention in which sixteendetectors are laid out on a section of seafloor above a subterraneanreservoir;

FIG. 9B compares the signal coverage of the prior art method and themethod of the invention;

FIG. 10 shows a graph plotting calculations of phases of differentcomponents of detector signals seen during a model electromagneticsurvey of the subterranean strata configurations shown in FIG. 4 at twodifferent source electromagnetic signal frequencies;

FIGS. 11A and 11B show graphs plotting differences in the phases shownin FIG. 10;

FIG. 12A shows a graph plotting calculations of differences in phasesbetween different components of detector signals seen during a modelelectromagnetic survey of several hydrocarbon-layer subterranean strataconfigurations with an electromagnetic source signal frequency of 0.5Hz;

FIG. 12B shows a graph plotting calculations of differences in phasesbetween different components of detector signals seen during a modelelectromagnetic survey of several hydrocarbon-layer subterranean strataconfigurations with an electromagnetic source signal frequency of 0.25Hz;

FIG. 12C shows a graph plotting calculations of differences in phasesbetween different components of detector signals seen during a modelelectromagnetic survey of several hydrocarbon-layer subterranean strataconfigurations with an electromagnetic source signal frequency of 1.0Hz;

FIG. 12D shows a graph plotting calculations of differences in phasesbetween different components of detector signals seen during a modelelectromagnetic survey of several hydrocarbon-layer subterranean strataconfigurations with an electromagnetic source signal frequency of 2.0Hz;

FIG. 13A shows a graph plotting calculations of phases of differentcomponents of detector signals seen during a model electromagneticsurvey of several uniform background subterranean strata configurations;

FIG. 13B shows a graph plotting differences in the phases shown in FIG.13A;

FIG. 14 shows a graph plotting calculations of maximum observeddifferences in phases between different components of detector signalsseen during a model electromagnetic survey of a hydrocarbon-layersubterranean strata configuration, as a function of hydrocarbon-layerresistivity, and for several electromagnetic source frequencies;

FIG. 15A shows a graph plotting calculations of phases of differentcomponents of detector signals seen during a model electromagneticsurvey of the subterranean strata configurations shown in FIGS. 3 and 4;

FIG. 15B shows a graph plotting differences in the phases shown in FIG.15A;

FIG. 16A shows a graph plotting calculations of phases of differentcomponents of detector signals seen during a model electromagneticsurvey of the subterranean strata configurations shown in FIGS. 3 and 4;and

FIG. 16B shows a graph plotting differences in the phases shown in FIG.16A;

DETAILED DESCRIPTION

A method of electromagnetic surveying for oil and other hydrocarbonreserves is described which does not require separate data acquisitionof the response of a subterranean strata configuration to inductivelyand galvanically coupled modes. The new method can be performed usingpre-existing survey equipment.

FIG. 2A schematically shows a surface vessel 14 undertaking EM surveyingof a subterranean strata configuration in a way that is suitable forcollecting survey data for carrying out the invention. The subterraneanstrata configuration includes an overburden layer 8, an underburdenlayer 9 and a hydrocarbon layer (or reservoir) 12. The surface vessel 14floats on the surface 2 of the seawater 4. A deep-towed submersiblevehicle 19 carrying a HED antenna 21 is attached to the surface vessel14 by an umbilical cable 16 providing an electrical, optical andmechanical connection between the deep-towed submersible vehicle 19 andthe surface vessel 14. The HED antenna broadcasts a sourceelectromagnetic signal into the seawater 4.

One or more remote detectors 25 are located on the seafloor 6. Eachdetector 25 includes an instrument packages 26, a detector antenna 24, afloatation device 28 and a ballast weight (not shown). The detectorantenna 24 measures a detector signal in response to EM fields inducedby the HED antenna in the vicinity of the detector 25, the amplitude ofthe detector signals is sensitive to resistivity variations in theunderlying strata configuration. The instrument package 26 records thedetector signals for later analysis. The detector antenna 24 in thisexample comprises two orthogonal dipole antennae arranged to detectfirst and second components of the electric field in a horizontal plane,i.e. one which is parallel to the seafloor 6.

The detectors record two (or three) orthogonal components of theseafloor electric field as raw data. The raw survey data are thenanalysed, after recovery of the detectors and transfer of the raw datainto a suitable computer. Initially a spectral analysis is performed toremove the component of the signal which corresponds to sourcetransmission, as is conventional. The survey data are then combined withsource and receiver navigation data, again as is conventional. Then thesurvey data are processed to rotate the electric fields from an‘instrument’ frame (i.e. components parallel to the receiver dipoles ofthe detector) to the ‘source’ frame (i.e. radial and azimuthalcomponents referenced to the source-receiver geometry). This is a newprocessing step specific to the present invention.

FIG. 2B is a schematic plan view detailing a polar-coordinate systemwhich is used to describe the principles of the new method. The originof the coordinate system is positioned at the centre of the HED antennashown in FIG. 2A, and zero-azimuth is aligned parallel to the dipoleaxis of the HED antenna, as indicated in FIG. 2B (the HED antenna inthis Figure not drawn to scale). In FIG. 2B, a single detector 25 isshown positioned at a range of R km from the origin, and at an azimuthof Φ°. The orthogonal dipole antennae comprising the detector antenna 24are arbitrarily oriented in the horizontal plane as indicated in thefigure.

Since the phase difference from the source of electromagnetic radiationfrom a horizontal dipole source is an azimuthally symmetric subterraneanstrata configuration is largely independent of azimuth Φ, phasemeasurements recorded at the detector antenna 24 are largely independentof the azimuthal position of the detector 25 shown in FIG. 2B. Thisallows phase data to be collected equally over a wider range ofsource-detector orientations than is possible with amplitude data, andany inaccuracies in the measurement of the azimuthal position of thedetector in the coordinate system shown in FIG. 2B have a lesser effect.

In the following examples, the two components of detected electricfield, also known as detector signal, for which the phase is measuredare a radial component and an azimuthal component. The radial componentis that component of the electric field resolved along a directionparallel to a line connecting the source location and the detectorlocation, and marked E_(ρ) in FIG. 2B. The azimuthal component is thatcomponent of the electric field resolved along a direction perpendicularto a line connecting the source location and the detector location andin a horizontal plane, and marked E_(Φ) in FIG. 2B. The components ofthe detected electric field along these directions is determined fromthe angular orientation of the orthogonal dipole antennae comprising thedetector antenna 24 relative to the line joining the source location andthe detector location. This can be easily determined using standardinstrumentation, such as, for example, active or passive sonar todetermine the relative positions of the source location and the detectorlocation, and a magnetic compass to determine the detector antennaorientation.

In order to show how the respective phases of two spatial components(e.g. radial ρ and azimuthal Φ components) of the electric field can beused to detect the presence of a subterranean hydrocarbon reservoir,numerical forward modelling of the kind described by Chave and Cox [11]is applied to different model subterranean strata configurations.

FIG. 3 shows in schematic vertical section a model backgroundsubterranean strata configuration. The configuration comprises a sectionof seafloor 106 beneath a 10 km depth of seawater 104. The seawater hasa resistivity of 0.31 Ωm. Beneath the seafloor 106 is a uniformhalf-space sedimentary structure with a resistivity of 1 Ωm, the lowresistivity being primarily due to aqueous saturation of pore spaces.This background subterranean strata configuration extends uniformlydownwards for an infinite extent. Also indicated in FIG. 3 are a HEDantenna 21, and a detector 25, such as those shown in FIG. 2A. Thedistance between the HED antenna and the detector (i.e. the range) is Rkm. The azimuthal position of the detector relative to the orientationof the HED antenna is arbitrary due to the insensitivity of the phasecomponent of the detected electric field signals to azimuth.

FIG. 4 shows in schematic vertical section a model hydrocarbon-layersubterranean strata configuration. A section of seafloor 106 liesbeneath a 10 km depth of seawater 104 which has a resistivity of 0.31Ωm. The strata configuration beneath the seafloor 106 comprises a 1 kmthick overburden layer 108, representing sediments, arranged above ahydrocarbon layer 112. The overburden layer 108 has a resistivity of 1Ωm, again, primarily due to aqueous saturation of pore spaces. Thehydrocarbon layer 112 is 0.1 km thick, and has a resistivity of 100 Ωm.The relatively high, resistivity of the hydrocarbon layer is due to thepresence of non-conducting hydrocarbon within pore spaces. Below thehydrocarbon layer 112 is a sedimentary underburden layer 109, which, asfor the overburden layer, has a resistivity of 1 Ωm. The underburdenlayer extends downwardly for an effectively infinite extent.Accordingly, except for the presence or absence of the hydrocarbon layer112, the background subterranean strata configuration of FIG. 3 and thehydrocarbon-layer subterranean strata configuration of FIG. 4 areidentical. A HED antenna 21 and a detector 25 are again shown as in FIG.3.

FIG. 5A shows a graph plotting the modelled phase θ of the radial andazimuthal components of the detected electric field for both thebackground subterranean strata configuration and the hydrocarbon-layersubterranean strata configuration models shown in FIGS. 3 and 4respectively as a function of range R. The phase is measured relative toa source electromagnetic signal transmitted by the HED antenna 21. Inthis example, the source electromagnetic signal is at a frequency of 0.5Hz. The radial and azimuthal components of the detected electric fieldfor the background subterranean strata configuration are marked θ_(ρ)^(B) and θ_(Φ) ^(B) respectively and the corresponding components of thedetected electric field for the hydrocarbon-layer subterranean strataconfiguration are marked θ_(ρ) ^(R) and θ_(Φ) ^(R) respectively. Theresults show that θ_(ρ) ^(B), θ_(Φ) ^(B), θ_(ρ) ^(R) and θ_(Φ) ^(R) alladvance steadily in phase with increasing range R. However, it is alsoclear that the rate of phase advance is less for the hydrocarbon-layersubterranean strata configuration than for the background subterraneanstrata configuration. In the case of the background subterranean strataconfiguration, the phase of both the radial and azimuthal componentsadvances at a rate of around 90° per km. In addition, at ranges beyondabout 2 km, the azimuthal component θ_(Φ) ^(B) consistently lags theradial component θ₉₂ ^(B) by around 25°. In the hydrocarbon-layersubterranean strata configuration, however, the behaviour is somewhatdifferent. Beyond around 5 km, the azimuthal component θ₁₀₁ ^(R) againlags the radial component θ_(ρ) ^(R) by around 25°, however the phase ofboth components advances at a rate of around only 10° per km. This issignificantly lower than that seen with the background subterraneanstrata configuration. Furthermore, at ranges between around 2 km and 5km, the difference in phase between azimuthal component θ_(Φ) ^(B) andthe radial component θ_(ρ) ^(R) varies significantly. A phase separationanomaly is seen which varies from close to 0° phase difference betweenthe radial and azimuthal components of detected electric field to amaximum of almost 60°.

FIG. 5B shows a graph plotting, for both the radial and azimuthalcomponents, the difference in phase θ^(R)-θ^(B) between thehydrocarbon-layer and background subterranean strata configurations as afunction of range R. The difference in the radial components is markedθ_(ρ) ^(R)-θ_(ρ) ^(B), and the difference in azimuthal components ismarked θ_(Φ) ^(R)-θ₁₀₁ ^(R). The different rates of phase advancementseen with the hydrocarbon-layer and background subterranean strataconfigurations is apparent in the negative gradient of the curves beyondaround 3 km. The differing behaviour at mid ranges (between around 2 kmand 5 km) is apparent from the separation of the curves over this range.

These differences in phase behaviour, namely the relatively slowadvancement in phase of both radial and azimuthal components when areservoir is present, and the strong range-limited variation in phasebetween the radial and azimuthal components seen at mid-ranges when thereservoir is present, provide two useful characteristics with which todetermine the presence or absence of a hydrocarbon layer within anotherwise uniform background.

For the practical application of controlled source electromagneticsurveying to hydrocarbon exploration, it is necessary that other commonsubterranean strata configurations do not lead to a behaviour similar tothat seen in the hydrocarbon-layer subterranean strata configurationmodel. In particular, it is important to be able to distinguish betweensubterranean strata configurations which include a thin hydrocarbonlayer and non-hydrocarbon containing subterranean strata configurationsthat have increasing resistivity with depth.

FIG. 6 shows in vertical section a highly schematic model of anon-hydrocarbon containing subterranean strata configuration. Thissubterranean strata configuration exhibits increasing resistivity withdepth, which is a common feature of many large scale sedimentarystructures. Due, for example, to increasing expulsion of conductingseawater with depth from rising overburden pressure. As with thebackground and hydrocarbon-layer subterranean strata configurationsdescribed above, in the non-hydrocarbon bearing subterranean strataconfiguration a section of seafloor 106 lies beneath a 10 km depth ofseawater 104. The strata beneath the seafloor 106 comprise a series ofsedimentary layers of increasing resistivity. A first layer 110 has auniform resistivity of 1 Ωm and a thickness of 1 km. A second layer 113has a uniform resistivity of 5 Ωm and a thickness of 1 km. A third layer114 has a uniform resistivity of 50 Ωm and a thickness of 1 km. Beneaththe third layer 114 is a fourth layer 116 which has a resistivity of 100Ωm and extends downwardly for an infinite extent. A HED antenna 21 and adetector 25 are again shown as in FIG. 3.

FIG. 7 shows a graph which is similar to and will be understood from thedescription of FIG. 5A above, but which also includes modelled curvesdetermined for the non-hydrocarbon bearing subterranean strataconfiguration. The modelled curves marked θ_(ρ) ^(B), θ_(Φ) ^(B), θ_(ρ)^(R), and θ_(Φ) ^(R) are the same as those shown in FIG. 5A. The curvesmarked θ_(ρ) ^(S) and θ_(Φ) ^(S) show the radial and azimuthalcomponents of the detected electromagnetic field seen with thenon-hydrocarbon bearing subterranean strata configuration.

The behaviour of the variation in phase of the detected electric fieldas a function of range beyond around 5 km is broadly similar for thehydrocarbon-layer subterranean strata configuration and thenon-hydrocarbon bearing subterranean strata configuration. There aremoderate differences in gradient between the models for the exampleshown, and in some circumstances this may allow the two subterraneanstrata configurations to be distinguished. (In other examples, thegradients are almost the same.) However, even if measurable, the valueof the gradient is likely to be a fairly unreliable indicator ofsubterranean strata configuration in practice. This is because differentabsolute values of resistivity, for instance a more or less resistivehydrocarbon layer in the hydrocarbon-layer subterranean strataconfiguration, or a more rapidly increasing resistivity with depth in anon-hydrocarbon bearing subterranean strata configuration, are likely tolead to changes in the observed gradients and cause confusion betweenthe two models.

However, at ranges between around 2 km and 5 km, there is nothing in thephase of the detected electric fields in response to the non-hydrocarbonbearing subterranean strata configuration which resembles the phaseseparation anomaly seen with the hydrocarbon-layer subterranean strataconfiguration. The phase behaviour seen with the non-hydrocarbon bearingsubterranean strata configuration much more closely resembles that ofthe background subterranean strata configuration across this range.Accordingly, it is the phase separation anomaly (and not the gradient)which provides the most appropriate indicator of subterranean strataconfiguration.

FIG. 8 shows a graph plotting the difference in the phase Δθ between theradial and azimuthal components of the detected electric field for thethree model subterranean strata configurations described above asfunction of range R. The curve marked Δθ^(B) in FIG. 8 represents thedifference between the curves marked θ_(Φ) ^(B) and θ_(Φ) ^(B) in FIG. 7(with negative values corresponding to the azimuthal component laggingthe radial component). The curves marked Δθ^(R) and Δθ^(S) in FIG. 8correspondingly represent the differences between the curves markedθ_(Φ) ^(R) and θ_(ρ) ^(R), and θ_(Φ) ^(S) and θ_(ρ) ^(S) in FIG. 7respectively.

The phase separation anomaly seen with the hydrocarbon-layersubterranean strata configuration (curve marked Δθ^(R) in FIG. 8) isapparent as a clear trough centred at a range of around 3.5 km. Themagnitude of the phase separation anomaly at this point is almost 60°.This difference in phase between the radial and azimuthal components ofthe detected electric field is about 30° more negative than the largestdifference seen with either the background or the non-hydrocarbonbearing subterranean strata configurations. With current technology, aphase differences between the radial and azimuthal components of thedetected electric field of 10° can be clearly resolved. Accordingly, thepresence or not of a trough similar to that seen in FIG. 8 is easilydetectable, and able to distinguish between a hydrocarbon-layersubterranean strata configuration of the type shown in FIG. 3, and themodel subterranean strata configurations shown in FIGS. 4 and 6.

By distributing a linear array of detectors along a section of seafloor,and at each one recording suitable raw data in response to a sourceelectromagnetic signal broadcast by a horizontal electromagnetic dipolesource, plots such as those shown in FIG. 8 can be generated from thephase information obtained from the raw data. The results of these plotscan then be used to indicate the type subterranean strata configurationbeneath a line joining the source and the detectors. Unlike previoussurvey methods, this can be done using a single dipole source andwithout the need to collect multiple data sets corresponding todifferent orientations of the source.

By distributing a planar array of detectors on a section of seafloor,and at each one recording raw data in response to a sourceelectromagnetic signal broadcast by a horizontal electromagnetic dipolesource, plots such as those shown in FIG. 8 can be generated for anumber of different directions once the phase information has beenextracted from the raw data. Because of the insensitivity of phase tothe azimuthal position of a detector with respect to the source dipoleaxis, the plots along each of the different directions achievable with aplanar array of detectors can be obtained simultaneously, irrespectiveof the dipole source orientation. This allows a thorough two- orthree-dimensional survey to be performed without even having to move thesource. This contrasts to previous methods where a relatively long andcomplicated tow of the dipole source is required to utilise all of thedetectors in a planar array, and then only with relatively low spatialsampling. Whilst all of the detectors in a planar array can be utilisedwithout moving the source, in practical surveys employing the newmethod, it is likely that the source will nonetheless be moved, such asshown in FIG. 2A. Each new source position provides an entire set ofuseful source-detector geometries, and so provides more comprehensivesampling of the subterranean strata configuration for a given number ofdetectors. In addition, by moving the source, surveys can be fullyperformed where the detectors are deployed over an area with acharacteristic scale larger than the range of distances over which phasemeasurements can be reliably used to indicate the presence of ahydrocarbon layer.

FIG. 9A shows in plan view an area of seafloor 6 to be surveyed andwhich is similar to that shown in the prior art FIG. 1. There aresixteen detectors 25 for recording the phase components described above.The detector are laid out in a square grid above a subterraneanreservoir 56. Other detector distributions could be used instead, suchas other grid shapes, or distributions that are not in a simple grid.(The constraints on detector placement patterns imposed by theamplitude-based methods of both Statoil and Sinha are therefore lifted.)The subterranean reservoir 56 has a boundary indicated by a heavy line58. The orientation of the subterranean reservoir is indicated by thecardinal compass points (marked N, E, S and W for North, East, South andWest respectively) indicated in the upper right of the figure. Toperform a survey using an embodiment of the new method, a source startsfrom location ‘A’ and is towed along a path indicated by the broken line120 to location ‘B’, which marks the end of the survey path. At mostpoints along the tow path, useful data can be collected from all of thedetectors. For example, when the source antenna is at the locationmarked by the reference numeral 57 in FIG. 9A, all sixteen of thedetectors 25 are able to collect reliable data. This contrasts to thecorrespondingly similar location shown in FIG. 1, again marked by thereference numeral 57, at which point no useful data can be collectedusing previous methods. Accordingly, the tow path shown in FIG. 9A,which is startlingly simple compared to that shown in FIG. 1, actuallyprovides a much greater amount of valid data. As noted above, with thenew method, it is only necessary to know the relative positions of thesource and detectors, and the orientation of each detector antenna suchthat the radial and azimuthal components of the detected electric fieldcan be geometrically resolved. Since the orientation of the antenna isnot critical, there is no need for the tow path 120 shown in FIG. 9A toclosely follow a pattern defined by the grid of detectors. In fact, itis preferable for the tow path to not align too closely with thenorth-south and east-west based detector grid, since for detectors in anend-on position (i.e. at an azimuth of 0° in the coordinate system shownin FIG. 2B), the amplitude of the azimuthal component of the detectedelectric field will be small for a dipole source, and the phase of thiscomponent more difficult to accurately establish. In source-detectororientations where either the radial or azimuthal components of thedetected electric field are small, other components of the detectedelectric field may be employed, for instance as described further below.

FIG. 9B is a graphical representation comparing the signal spread of thenew phase method to the old inline/broadside amplitude method. Theexample reservoir 56 bounded by the perimeter 58 is shown. The dipolesource is at an arbitrary location 57 within the reservoir with the HEDantenna axis aligned W-E. In the old method, good quality inlineamplitude data is only collectable within a narrow angular range 64indicated by W-E dark shading in the figure, and good quality broadsideamplitude data is only collectable within a narrow angular range 62indicated by the N-S dark shading in the figure. The angular ranges 62and 64 need to be narrow to ensure that one of the inductive andgalvanic signal components dominates over the other. Data collected bydetectors in the four main quadrants 66 is essentially bad data to berejected from analysis. On the other hand, in the new method, thesituation is reversed. The broad quadrants 66 become the regions overwhich good quality data is collected, since they are the regions inwhich phase can be fully decomposed into the radial and azimuthal signalcomponents needed for the phase-difference anomaly measurement, whereasthe dark areas 62, 64 are angular areas where the collected data becomesunreliable since the magnitude of one of the radial and azimuthal signalcomponents is likely to become too small causing signal-to-noiseproblems.

The modelled phase responses shown in FIGS. 5A, 5B, 7 and 8 were allcalculated for a horizontal electromagnetic dipole source transmitting asource electromagnetic signal at a frequency of 0.5 Hz.

FIG. 10 shows a graph plotting the modelled phase θ of the radial andazimuthal components of the detected electric field for thehydrocarbon-layer subterranean strata configuration model shown in FIG.3 as a function of R for two different frequencies of sourceelectromagnetic signal. The modelled radial and azimuthal components ofthe detected electric field seen in response to a dipole sourcetransmitting at a frequency of 2 Hz are marked θ_(ρ) ^(2Hz) and θ_(Φ)^(2Hz) respectively, and the modelled radial and azimuthal components ofthe detected electric field seen in response to a dipole sourcetransmitting at a frequency of 0.25 Hz are marked θ_(ρ) ^(0.25Hz) andθ_(Φ) ^(0.25Hz) respectively. These curves, and also comparison with themodelled radial and azimuthal components of the detected electric fieldseen in response to a dipole source transmitting at a frequency of 0.5Hz, marked θ_(ρ) ^(R) and θ_(Φ) ^(R) in FIG. 5A, indicate a frequencydependence to the characteristics of the phase separation anomalyindicative of a buried hydrocarbon layer. Towards higher frequencies,the phases of the radial and azimuthal components advance faster than atlower frequencies, and the scale over which the phase separation anomalycharacteristic of a hydrocarbon layer's presence occurs is also seen tobe frequency dependent.

FIG. 11A shows a graph plotting the difference in the phase Δθ betweenthe radial and azimuthal components of the detected electric field forthe reservoir and background model subterranean strata configurationsdescribed above as function of range R, in response to a dipole sourcetransmitting at a frequency of 2 Hz. The curve marked Δθ^(R) in FIG. 11Arepresents the difference between the curves marked θ_(Φ) ^(2Hz) andθ_(ρ) ^(2Hz) in FIG. 10 (with positive values corresponding to theradial component lagging the azimuthal component). The curve markedΔθ^(B) represents the corresponding data for the background modelsubterranean strata configuration.

FIG. 11B is shows a graph plotting the difference in the phase Δθbetween the radial and azimuthal components of the detected electricfield for the reservoir and background model subterranean strataconfigurations described above as function of range R, in response to adipole source transmitting at a frequency of 0.25 Hz. The curve markedΔθ^(R) in FIG. 11B represents the difference between the curves markedθ_(Φ) ^(0.25Hz) and θ_(ρ) ^(0.25Hz) in FIG. 10 (with positive valuescorresponding to the radial component lagging the azimuthal component).The curve marked Δθ^(B) represents the corresponding data for thebackground model subterranean strata configuration.

It is clear from FIGS. 8, 11A and 11B, that the range over which thephase separation anomaly occurs is smaller at higher frequencies. At 2Hz (see FIG. 11A), the phase separation anomaly is centred at a range ofaround 3 km and occurs over a characteristic range of about 1 km. At 0.5Hz (see FIG. 8), the phase separation anomaly is centred at a range ofaround 3 km and occurs over a characteristic range of about 2 km. At0.25 Hz (see FIG. 11B), the phase separation anomaly is centred at arange of around 3 km and occurs over a characteristic range of about 3km. For all frequencies, the maximum phase separation seen with thehydrocarbon-layer subterranean strata configuration is about 30° morenegative than the phase difference that would be seen if the hydrocarbonlayer were not present. This indicates that the presence of ahydrocarbon layer can be detected using a range of frequencies, each ofwhich acts a probe of the subterranean strata configuration operatingover a slightly different spatial scale.

FIG. 12A is a graph showing the effect of differing overburdenthicknesses. The graph plots the difference in the phase Δθ between theradial and azimuthal components of the detected electric field forseveral hydrocarbon-layer subterranean strata configurations withdifferent overburden thicknesses as function of range R. In thisexample, the source electromagnetic signal is at a frequency 0.5 Hz.Curves are plotted for different hydrocarbon-layer subterranean strataconfigurations which, while otherwise similar to the hydrocarbon-layersubterranean strata configuration shown in FIG. 4, have overburdenthicknesses of 0.25 km, 0.5 km, 1.0 km, 1.5 km and 2.5 km. The curvescorresponding to each different overburden thickness are correspondinglymarked in the figure. The curve marked 1.0 in FIG. 12A is identical tothe curve marked Δθ^(R) in FIG. 8 since the overburden thickness in themodel shown in FIG. 3 (and used for the modelling shown in FIG. 8) is1.0 km. The curve marked Δθ^(B) in FIG. 12A is similar to and will beunderstood from the similarly marked curve in FIG. 8. For each of thecurves corresponding to different overburden thicknesses, the magnitudeof the phase separation anomaly is roughly similar, varying from about55° with an overburden thickness of 0.25 km to about 65° with anoverburden thickness of 2.5 km. Accordingly, the method is equally ableto detect a thin hydrocarbon layer at a range of depths beneath the seafloor. It is also apparent that the range at which the phase separationanomaly is maximum increases with increasing overburden thickness. Thissensitivity of the range of maximum phase separation anomaly tooverburden thickness can allow the depth of a reservoir to be determinedwith appropriate inversion modelling and suitable data coverage.

FIGS. 12B, 12C and 12D are similar to and will be understood from FIG.12A. However, FIGS. 12B, 12C and 12D show the response of differentoverburden thicknesses to different frequencies of sourceelectromagnetic signal. FIG. 12B shows the response to a sourceelectromagnetic signal at a frequency 0.25 Hz, FIG. 12C shows theresponse to a source electromagnetic signal at a frequency 1.0 Hz, FIG.12B shows the response to a source electromagnetic signal at a frequency2 Hz. It can be seen that the phase separation anomaly is detectablewith a range of frequencies over a range of overburden thicknesses. Themagnitude of the phase separation anomaly is broadly similar at each ofthe different frequencies shown. As seen previously, the range overwhich the phase separation is apparent narrows with increasingfrequency.

FIG. 13A is a graph showing the effect of differing backgroundresistivity. The graph plots the modelled phase θ of the radial andazimuthal components of the detected electric field for a backgroundsubterranean strata configuration similar to that shown in FIG. 3, butwith different resistivity values for the uniform subterranean strata,as a function of range R. In this example, the source electromagneticsignal is at a frequency 0.5 Hz. The phase of the radial and azimuthalcomponents of the detected electric field are calculated for resistivityvalues of 1 Ωm, 5 Ωm, 15 Ωm, 50 Ωm and 200 Ωm, as marked in the figure.For each resistivity value the phase of the radial component of thedetected electromagnetic field is shown as a dashed line, and the phaseof the azimuthal component is shown as a solid line. The pair of curvescorresponding to the 1 Ωm resistivity value are identical to the curvesmarked θ_(ρ) ^(B) and θ_(Φ) ^(B) in FIG. 5A. It is clear that theresistivity value for the subterranean strata in a uniform modelcontaining no hydrocarbon reservoir has a significant effect on thedetected phase. The rate of advancement of phase with range, for boththe radial and azimuthal components of the detected electric field,falls with increasing background resistivity. For example, at aresistivity of around 15 Ωm, the rate of phase advancement with range isabout 15° per km. This is similar to the rate of phase advancement seenwith the reservoir subterranean strata configuration model and plottedin FIG. 5A for ranges beyond around 5 km. This again demonstrates howabsolute values of phase for each of the radial and azimuthal componentscan be an unreliable indicator of the likely presence of a hydrocarbonlayer within an otherwise uniform resistivity background.

FIG. 13B shows a graph plotting the difference in phase Δθ between theradial and azimuthal components of the detected electric field for eachof the different resistivity value background subterranean strataconfigurations shown in FIG. 13A. Each curve is appropriately markedaccording to the resistivity value of the model to which it corresponds.The curve marked 1 Ωm is identical to the curve marked Δθ^(B) in FIG. 8.While the characteristic difference in phase for the radial andazimuthal components is depends on the resistivity of the uniformsubterranean strata, none of the curves shown in FIG. 13B display arange limited phase separation anomaly which, as seen in FIG. 8, isindicative of the presence of a buried hydrocarbon layer. It is theresistivity contrast between a buried hydrocarbon layer and an otherwiseuniform background which gives rise to the phase separation anomaly.Accordingly, by forming the phase difference between the radial andazimuthal components of the detected electric field in the mannerdescribed above, a hydrocarbon-layer containing subterranean strataconfiguration remains clearly distinguishable from a range of uniformsubterranean strata configurations of differing resistivities.

FIG. 14 is a graph showing the effect of hydrocarbon-layerresistivities. The graph plots the largest difference in phase max(Δθ)between the radial and azimuthal components of the detected electricfield as a function of differing hydrocarbon-layer resistivity P, in ahydrocarbon-layer subterranean strata configuration which is otherwisesimilar to that shown in FIG. 3. Curves are shown for sourceelectromagnetic signal frequencies of 0.25 Hz, 0.5 Hz, 1 Hz and 2 Hz, asindicated in the figure. For example, with a source electromagneticsignal frequency of 0.5 Hz, and a hydrocarbon-layer resistivity of 100Ωm, the largest difference in phase between the radial and azimuthalcomponents of the detected electric field is about −58°. This particularvalue corresponds to the minimum seen in FIG. 8 for the curve markedΔθ^(R). Typical hydrocarbon-layer resistivities are between a few tensof Ωm and a few hundreds of Ωm. It can be seen from FIG. 14 that for alltypical values of hydrocarbon-layer resistivity, a difference in phasebetween the radial and azimuthal components of the detected electricfield of at least 30° is seen for all source electromagnetic signalfrequencies. At lower frequencies, it is even higher.

This demonstrates that the above described method is able to detecthydrocarbon layers with different resistivities, and using a range ofsource electromagnetic signal frequencies.

It can also be seen from FIG. 14 that the maximum phase differencebetween the radial and azimuthal components of the detected field isgreatest for a hydrocarbon layer with a resistivity of about 50 Ωm. Atresistivities above and below this value, a decreasing maximum phasedifference is seen.

It has thus been demonstrated that the presence of a hydrocarbon layerin a subterranean strata configuration can be detected by observing thephase difference between radial and azimuthal components of detectedelectric field in response to a source electromagnetic signal from ahorizontal electric dipole source. This has been shown to work over awide range of source frequencies, for differing depths of burial of ahydrocarbon layer and for different subterranean strata configurationresistivity values.

ALTERNATIVE EMBODIMENTS

Whilst in the above examples the radial and azimuthal components havebeen considered, similar techniques can also be employed using differentcomponents of the detected electromagnetic field. For instance, if thedetectors 25 shown in FIG. 2 were configured to also record the phase ofthe vertical component of the detected electric field (i.e.perpendicular to both the radial and azimuthal directions), the verticalcomponent could be used in combination with another component to probe asubterranean strata configuration.

FIG. 15A shows a graph plotting the modelled phase θ of the vertical andazimuthal components of the detected electric field for both thebackground subterranean strata configuration and the hydrocarbon layersubterranean strata configuration models shown in FIGS. 3 and 4respectively as a function of R. The phase is measured relative to asource electromagnetic signal transmitted by the HED antenna. In thisexample, the source electromagnetic signal is at a frequency 0.5 Hz. Thevertical and azimuthal components of the detected electric field for thebackground subterranean strata configuration are marked θ_(z) ^(B) andθ_(Φ) ^(B) respectively and the corresponding components of the detectedelectric field for the hydrocarbon-layer subterranean strataconfiguration are marked θ_(z) ^(R) and θ_(Φ) ^(R) respectively. Exceptfor showing the vertical rather than the radial components of thedetected electric field, this figure directly corresponds to FIG. 5A.

FIG. 15B shows a graph plotting the difference in the phase Δθ betweenthe vertical and azimuthal components of the detected electric field forthe two model subterranean strata configurations included in FIG. 15A.The curve marked Δθ^(B) in FIG. 15B represents the difference betweenthe curves marked θ_(Φ) ^(B) and θ_(z) ^(B) in FIG. 15A (with negativevalues corresponding to the vertical component lagging the azimuthalcomponent). The curve marked Δθ^(R) FIG. 15B correspondingly representsthe difference between the curves marked θ_(Φ) ^(R) and θ_(z) ^(R) inFIG. 15A.

FIG. 16A shows a graph plotting the modelled phase θ of the vertical andradial components of the detected electric field for both the backgroundsubterranean strata configuration and the hydrocarbon-layer subterraneanstrata configuration models shown in FIGS. 3 and 4 respectively. Thephase is measured relative to a source electromagnetic signaltransmitted by the HED antenna. In this example, the sourceelectromagnetic signal is at a frequency 0.5 Hz. The vertical and radialcomponents of the detected electric field for the backgroundsubterranean strata configuration are marked θ_(z) ^(B) and θ_(ρ) ^(B)respectively and the corresponding components of the detected electricfield for the hydrocarbon-layer subterranean strata configuration aremarked θ_(z) ^(R) and θ_(ρ) ^(R) respectively. Except for showing thevertical rather than the azimuthal components of the detected electricfield, this figure directly corresponds to FIG. 5A.

FIG. 16B shows a graph plotting the difference in the phase Δθ betweenthe vertical and radial components of the detected electric field forthe two model subterranean strata configurations included in FIG. 16A.The curve marked Δθ^(B) in FIG. 16B represents the difference betweenthe curves marked θ_(ρ) ^(B) and θ_(z) ^(B) in FIG. 16A (with negativevalues corresponding to the vertical component lagging the radialcomponent). The curve marked Δθ^(R) FIG. 18B correspondingly representsthe difference between the curves marked θ_(ρ) ^(R) and θ_(z) ^(R) inFIG. 18A.

FIGS. 15B and 16B both indicate that the difference in phase between thevertical component of detected electric field and either of theazimuthal or radial components is also sensitive to the presence of ahydrocarbon layer in an otherwise uniform background subterranean strataconfiguration. The phase difference seen between the vertical andazimuthal components displays both a negative and a positive lobecompared to the background subterranean strata configuration with acrossover at a range of around 4 km. This would be particularly usefulindicator for use in survey areas where background resistivity is poorlyconstrained. The qualitative behaviour of the phase of the verticalcomponent of the detected electric field is approximately similar tothat of the azimuthal component. However, a larger phase separation isseen when comparing the radial component with the azimuthal componentthan when comparing the radial component with the vertical component.Accordingly, the azimuthal component will generally be preferred whencomparison is made with the radial component, unless, for instance, themagnitude of the azimuthal component is small, for example, where adetector is very close to an end-on orientation.

In the above description, and in FIGS. 5A, 7, 10, 13A, 15A and 15B, theabsolute phase of various components of the detected electric field hasbeen considered relative to the source electromagnetic signal phase.However, in practice, since it is the relative between differentcomponents of the detected electric field seen at the detector which isindicative of the presence of a hydrocarbon layer, the detectedcomponents may be directly compared without reference to the phase ofthe source electromagnetic signal.

Finally it will be understood that the invention is equally applicableto surveying of freshwater, for example large lakes, so that referencesto seafloor, seawater etc. should not be regarded as limiting.

SUMMARY

It has been demonstrated how a phase separation anomaly occurs inresponse to a hydrocarbon layer which is not seen with a backgroundsubterranean strata configuration. This allows the detection ofsubterranean hydrocarbon reservoirs and hydrocarbon bearing layers. Thetechnique has many advantages over previous methods, for example:

Previous techniques based on comparison of amplitude measurementsrequire the collection of both end-on and broadside data for eachreceiver to be reliable. This requires multiple orthogonal survey towpaths (see FIG. 1). Using a technique such as described above, a surveymay be completed more thoroughly with a much shorter and less complextow path (see FIG. 9A).

The orthogonal towpaths required by previous methods lead to sampling ofdifferent parts of a target structure. Because only singlesource-receiver pairs are required for a phase-based detection of thereservoir, there is reduced interpretational ambiguity arising from thedimensionality of the target structure. This overcomes the limitation ofthe prior art Sinha method in which in-line data from a given receiverwill come from one source position and the corresponding broadside dataat the same range will generally come from a different source location.This means that the structure sampled between the source and detectorfor the compared in-line and broadside data will not be the same. Withthe new method, this problem does not arise, since all the data iscollected from a single source position, so both phase components in theprocessed signal are derived from sampling the same structure.

The above described technique is almost independent of detector azimuthrelative to a source's dipole axis. Since the method is less dependenton the orientation of the source, geometry-related errors are muchreduced. In order to decompose the detector signals into radial andazimuthal (or whichever components are desired) it is only necessary toknow the relative positions of the source and detectors, and theorientation of the detector antenna. These can be easily determinedusing existing technology.

The phase separation seen above is range limited and can be controlledby varying the frequency of the electromagnetic source. If the sourcewere to broadcast at several discrete frequencies (either by employingmultiple source antenna or a tuned source for example) improved verticalresolution can be achieved.

For a particular source dipole transmission frequency, the rangedependence of phase separation can be used to indicate the depth to theresistive layer.

Phase data are relatively insensitive to structures which are local tothe receiver.

REFERENCES

-   [1] Sinha, M. C., Patel, P. D., Unsworth, M. J., Owen, T. R. E. &    MacCormack, M. R. G. An active source electromagnetic sounding    system for marine use. Mar. Geophys. Res., 12, 1990, 59-68.-   [2] Evans, R. L., Sinha, M. C., Constable, S. C. & Unsworth, M. J.    On the electrical nature of the axial melt zone at 13° N on the East    Pacific Rise. J. Geophys. Res., 99, 1994, 577-588-   [3] Edwards, R. N., Law, K. L., Wolfgram, P. A., Nobes, D. C.,    Bone, M. N., Trigg, D. F. & DeLaurier, J. M., First result of the    MOSES experiment: Sea sediment conductivity and thickness    determination, Bute Inlet, Columbia, bu magnetometric offshore    electrical sounding, Geophyics, 50, 1985, 153-161-   [4] WO 00/13046 A1-   [5] WO 01/57555 A1-   [6] Eidesmo, T., Ellingsrud, S., MacGregor, L. M., Constable, S.,    Sinha, M. C., Johansen, S, Kong, F-N & Westerdahl, H., Sea Bed    Logging (SBL), a new method for remote and direct identification of    hydrocarbon filled layers in deepwater areas, First Break, 20, 2002,    144-152.-   [7] Ellingsrud, S., Sinha, M. C, Constable, S., MacGregor, L. M.,    Eidesmo, T. & Johansen, S., Remote sensing of hydrocarbon layers by    sea-bed logging (SBL): Results from a cruise offshore Angola, The    Leading Edge, submitted 2002.-   [8] MacGregor, L. M. & Sinha, M. C. Use of marine controlled source    electromagnetic sounding for sub-basalt exploration. Geophysical    Prospecting, 48, 2000, 1091-1106.-   [9] WO 02/14906 A1-   [10] MacGregor, L. M., Constable, S. C. & Sinha, M. C. The RAMESSES    experiment III: Controlled source electromagnetic sounding of the    Reykjanes Ridge at 57° 45′ N. Geophysical Journal International,    135, 1998, 773-789.-   [11] Chave, A. D. & Cox, C. S., Controlled electromagnetic sources    for measuring electrical conductivity beneath the oceans, 1. Forward    problem and model study. J. Geophys. Res., 87, 1982, 5327-5338-   [12] Martin C. Sinha, “Controlled source EM sounding: Survey design    considerations for hydrocarbon applications”, LITHOS Science Report    April 1999, 1, 95-101-   [13] GB 2382875 A

1. An electromagnetic survey method for surveying an area that isthought or is known to contain a subterranean hydrocarbon reservoir,comprising: transmitting a source electromagnetic signal from a sourcelocation; detecting a detector signal at a detector location in responsethereto; obtaining survey data indicative of phase difference betweenfirst and second components of the detector signal resolved along firstand second directions respectively; and forming the phase differencebetween the first and second components.
 2. The survey method of claim1, wherein the first and second components are radial and azimuthal withreference to the source location-receiver location geometry.
 3. Thesurvey method of claim 1, wherein the first and second components arevertical and azimuthal with reference to the source location-receiverlocation geometry.
 4. The survey method of claim 1, wherein the firstand second components are vertical and radial with reference to thesource location-receiver location geometry.
 5. The survey method ofclaim 1, further comprising obtaining survey data indicative of phase ofa third component of the detector signal resolved along a thirddirection orthogonal to the first and second directions.
 6. The surveymethod of claim 5, wherein the first and second and third components arevertical, radial and azimuthal with reference to the sourcelocation-receiver location geometry.
 7. The survey method of claim 1,wherein the first and second directions are orthogonal.
 8. The surveymethod of claim 1, wherein the source electromagnetic signal isbroadcast from an antenna mounted on a submersible vehicle which istowed over the survey area to move the source location.
 9. The surveymethod of claim 1, wherein the source location is fixed.
 10. The surveymethod of claim 1, wherein the source electromagnetic signal is emittedat different frequencies to obtain survey data at a plurality ofdifferent frequencies.
 11. The survey method of claim 1, wherein thesource electromagnetic signal is emitted at a frequency of between 0.01Hz and 10 Hz.
 12. A method of analysing results from an electromagneticsurvey of an area that is thought or known to contain a subterraneanhydrocarbon reservoir, comprising: providing survey data indicative ofphase difference between first and second components of a detectorsignal resolved along first and second directions respectively.extracting the phase differences from the survey data; and determining ametric from the phase differences that is predictive of the presence orabsence of hydrocarbon.
 13. The analysis method of claim 12, wherein thefirst and second components are radial and azimuthal with reference tothe source location-receiver location geometry.
 14. The analysis methodof claim 12, wherein the first and second components are vertical andazimuthal with reference to the source location-receiver locationgeometry.
 15. The analysis method of claim 12, wherein the first andsecond components are vertical and radial with reference to the sourcelocation-receiver location geometry.
 16. The analysis method of claim12, further comprising obtaining survey data indicative of phase of athird component of the detector signal resolved along a third directionorthogonal to the first and second directions.
 17. The analysis methodof claim 16, wherein the first and second and third components arevertical, radial and azimuthal with reference to the sourcelocation-receiver location geometry.
 18. The analysis method of claim12, wherein the first and second directions are orthogonal.
 19. Theanalysis method of claim 18, wherein the phase differences are extractedby rotationally transforming the survey data from an instrument frame toa source frame.
 20. A computer program product bearing machine readableinstructions for implementing the method of claim
 12. 21. A method ofplanning an electromagnetic survey of an area that is thought or knownto contain a subterranean hydrocarbon reservoir, comprising: creating amodel of the area to be surveyed including a seafloor, a rock formationcontaining a postulated hydrocarbon reservoir beneath the seafloor, anda body of water above the seafloor; setting values for depth below theseafloor of the postulated hydrocarbon reservoir and resistivitystructure of the rock formation; performing a simulation of anelectromagnetic survey in the model; and obtaining from the model phasedifferences between first and second components of a detector signalresolved along first and second directions respectively.
 22. Theplanning method of claim 21, wherein the first and second components aretwo of radial, vertical and azimuthal with reference to the sourcelocation-receiver location geometry.
 23. The planning method of claim21, further comprising: repeating the simulation for a number ofdistances between a source and a detector and frequencies in order toselect optimum surveying conditions in terms of source-to-detectordistance for probing the hydrocarbon reservoir.
 24. A computer programproduct bearing machine readable instructions for implementing theplanning method of claim 21.